Method for fabricating a focal plane array for thermal imaging system

ABSTRACT

A hybrid thermal imaging system (20, 120) often includes a focal plane array (30, 130), a thermal isolation structure (50, 150) and an integrated circuit substrate (60, 160). The focal plane array (30, 130) includes thermal sensitive elements (42, 142) formed from a pyroelectric film layer (82), such as barium strontium titanate (BST). One side of the thermal sensitive elements (42, 142) may be coupled to a contact pad (62, 162) disposed on the integrated circuit substrate (60, 160) through a mesa strip conductor (56, 150) of the thermal isolation structure (50, 150). The other side of the thermal sensitive elements (42, 142) may be coupled to an electrode (36, 136). The various components of the focal plane array (30, 130) may be fabricated from multiple heterogeneous layers (74, 34, 36, 82, 84) formed on a carrier substrate (70).

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/368,066filed Jan. 3, 1995, entitled Method of Fabricating A Focal Plane Arrayfor Hybrid Thermal Imaging System now U.S. Pat. No. 5,644,838, and isrelated to application Ser. No. 08/229,497, entitled Thermal ImagingSystem With Integrated Thermal Chopper and Method now U.S. Pat. No.5,486,698, application Ser. No. 08/281,711, entitled Thermal ImagingSystem With A Monolithic Focal Plane Array now U.S. Pat. No. 5,512,748,and application Ser. No. 08/368,067, entitled Monolithic ThermalDetector with Pyroelectric Film and Method now U.S. Pat. No. 5,602,043.These applications are assigned to the same assignee.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method for fabricating multipleheterogeneous layers including a relatively thick pyroelectric filmlayer, and more particularly, to a hybrid thermal imaging system havingan integrated circuit substrate with a thermal isolation structurecoupled to a focal plane array formed from the multiple heterogeneouslayers.

BACKGROUND OF THE INVENTION

One common application for thermal sensors is in thermal (infrared)imaging device such as night vision equipment. One such class of thermalimaging devices includes a focal plane array of infrared detectorelements or thermal sensors having pyroelectric material. The focalplane array and its associated thermal sensors are often coupled to anintegrated circuit substrate with a corresponding array of contact padsand a thermal isolation structure disposed between the focal plane arrayand the integrated circuit substrate. The thermal sensors define therespective picture elements or pixels of the resulting thermal image.

One type of thermal sensor includes a thermal sensitive element formedfrom pyroelectric material which exhibits a state of electricalpolarization and/or change in dielectric constant dependent upontemperature changes of the pyroelectric material in response to incidentinfrared radiation. An infrared absorber and common electrode assemblyare often disposed on one side of the thermal sensitive elements. Asensor signal electrode is generally disposed on the opposite side ofeach thermal sensitive element. The infrared absorber and commonelectrode assembly typically extends across the surface of the focalplane array and is attached to the thermal sensitive elements. Eachthermal sensitive element generally has its own separate sensor signalelectrode. Each infrared detector element or thermal sensor may bedefined in part by the infrared absorber and common electrode assemblyand the respective sensor signal electrode. The common electrode and thesensor signal electrode constitute capacitive plates. The pyroelectricmaterial constitutes a dielectric or insulator disposed between thecapacitive plates.

For some thermal sensors barium strontium titanate (BST) may be used toform the thermal sensitive element for the resulting thermal sensors.The starting place for fabricating such thermal sensitive elements istypically a wafer of barium strontium titanate or other suitablepyroelectric material having a diameter of four inches and anapproximate thickness of 0.1 inches. Various grinding and/or polishingprocesses are frequently used to reduce the thickness of the BST waferto approximately 0.001 inches or less.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages and problemsassociated with previous thermal imaging systems having thermal sensorsformed from pyroelectric material have been substantially reduced oreliminated. The present invention allows fabricating a focal plane arrayfor a hybrid thermal imaging system from multiple heterogeneous layersincluding a relatively thick pyroelectric film layer. The resultingfocal plane array and associated thermal sensors may be coupled with athermal isolation structure disposed on an integrated circuit substrateusing conventional bump bonding techniques or other techniques forfabricating hybrid solid state systems. One embodiment of the presentinvention may include fabricating an infrared absorber and commonelectrode assembly, thermal sensitive elements, and respective sensorsignal electrodes using thin film preparation techniques associated withphotolithography.

Important technical advantages of one embodiment may include growing apyroelectric film having the desired thickness for use in fabricatingthermal sensors for a thermal imaging system. The use of suchpyroelectric film substantially reduces costs associated with polishingand grinding bulk pyroelectric materials to produce thermal sensitiveelements having the desired thickness. By growing a layer ofpyroelectric film with approximately the desired dimensions for theresulting thermal sensors, both fabrication costs and waste ofpyroelectric material are substantially reduced. One aspect of thepresent invention includes eliminating the need for reticulation bylaser or ion milling of pyroelectric material to produce the desiredthermal sensitive elements.

Another aspect of the present invention includes growing a layer ofpyroelectric film on a carrier substrate which is chemically compatiblewith the pyroelectric film. One or more separation layers may bedisposed between the substrate and the pyroelectric film layer.Depending upon the sensor signal flow path associated with the resultingthermal sensor one or more layers of electrically conductive materialmay also be disposed on the pyroelectric film layer. For someapplications, a layer of infrared absorbing material may be formed onthe substrate prior to growing the pyroelectric film layer. Theresulting multiple layers of infrared absorbing material, pyroelectricfilm and electrically conductive material may be processed using variousetching and/or photolithographic techniques to produce a focal planearray having a plurality of thermal sensors for coupling with anintegrated circuit substrate and it associated thermal isolationstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation in elevation and in section withportions broken away of a hybrid thermal imaging system having a focalplane array formed from multiple heterogeneous layers incorporating oneembodiment of the present invention and an integrated circuit substratewith a thermal isolation structure disposed therebetween;

FIG. 2 is a plan view of a carrier substrate satisfactory for growing apyroelectric film layer and other heterogeneous layers associated withthe focal plane array of FIG. 1 in accordance with one aspect of thepresent invention;

FIG. 3 is a drawing in section taken along lines 3--3 of FIG. 2;

FIG. 4 is a schematic representation in section with portions brokenaway showing the pyroelectric film layer and other layers associatedwith the focal plane array of FIG. 1 as grown on the substrate of FIGS.2 and 3;

FIG. 5 is a schematic representation in section with portions brokenaway showing an additional layer of electrically conductive materialdisposed on the pyroelectric film layer of FIG. 4;

FIG. 6 is a schematic representation in section with portions brokenaway showing an intermediate step during fabrication of the focal planearray in FIG. 1; and

FIG. 7 is a schematic representation in elevation and in section withportions broken away of a hybrid thermal imaging system having a focalplane array formed from multiple heterogeneous layers incorporatinganother embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention and its advantages are bestunderstood by referring to FIGS. 1 through 7 of the drawings, likenumerals being used for like and corresponding parts of the variousdrawings.

Some infrared detectors and thermal imaging systems are based uponeither the generation of a change in voltage due to a change intemperature resulting from incident infrared radiation striking athermal sensor or the generation of a change in voltage due to aphoton-electron interaction within the material used to form the thermalsensor. This latter effect is sometimes called the internalphotoelectric effect. Other infrared detectors and thermal imagingsystems are based upon the change in resistance of a thin conductorcaused by the heating effect of incident infrared radiation. Suchinfrared detectors are sometimes referred to as bolometers.

Thermal imaging systems constructed in accordance with the teachings ofthe present invention preferably function based upon the generation of achange in voltage due to a change in temperature of pyroelectricmaterial resulting from incident infrared radiation. However, thepresent invention may be used with other types of thermal imagingsystems including bolometers.

Thermal imaging systems or infrared detectors 20 and 120, which will bedescribed later in more detail, may sometimes be referred to as uncooledinfrared detectors. The various components of thermal imaging systems 20and 120 are preferably contained within an associated housing (notshown) in a vacuum environment. Alternatively an environment of lowthermal conductivity gas may be satisfactory for some applications.

Thermal imaging system 20 may be described as a hybrid solid statesystem formed by mounting focal plane array 30 with thermal isolationstructure 50 on integrated circuit substrate 60. Focal plane array 30preferably includes a plurality of thermal sensors 40. Focal plane array30 may be both electrically and mechanically coupled with integratedcircuit substrate 60 by thermal isolation structure 50 to produce athermal image in response to incident infrared radiation striking focalplane array 30.

The components of focal plane array 30 include infrared absorber andcommon electrode assembly 32 and a plurality of thermal sensors 40.Infrared absorber and common electrode assembly 32 may further compriseone or more layers of optical coating 34 forming a tuned cavity fromdielectric material such as parylene or zirconium oxide (ZrO₂) andcommon electrode 36. For some applications layer 34 may include multiplelayers depending upon the specific wavelength or wavelengths of infraredradiation that thermal sensors 40 are designed to detect.

Common electrode 36 may perform several important functions such asincreasing the interaction of incident infrared radiation with opticalcoating 34. Common electrode 36 is preferably electrically conductive toform a portion of the sensor signal flowpath which supplies bias voltageto thermal sensors 40. Also, common electrode 36 preferably has lowthermal conductivity to minimize transfer of heat energy between thermalsensitive elements 42 in each thermal sensor 40. Common electrode 36 maybe formed from materials such as nickel chrome, ruthenium (Ru),ruthenium oxide (RuO₂), lanthanum strontium cobalt oxide (LSCO) orplatinum which have appropriate values of thermal and electricalconductivity. The reflectivity of such metals also cooperates withoptical coating 34 to enhance the absorption of incident infraredradiation. For other embodiments of the present invention, materialsother than metal which have the desired characteristics of electricaland thermal conductivity may be used to form common electrode 36. Thepresent invention is not limited to using only metal for commonelectrode 36.

Each thermal sensor 40 may include a thermal sensitive element 42 whichis preferably formed from pyroelectric material. One side of eachthermal sensitive element 42 is preferably attached to common electrode36. Sensor signal electrodes 44 are attached to the opposite side ofrespective thermal sensitive elements 42. For some applications thermalsensitive elements 42 may be formed from pyroelectric material such asbarium strontium titanate (BST).

Thermal isolation structure 50 includes a plurality of mesa-typestructures 52 disposed on integrated circuit substrate 60 adjacent torespective contact pads 62. Thermal isolation structure 50 may be usedto provide mechanical support during bonding of focal plane array 30with integrated circuit substrate 60 and to thermally insulate focalplane array 30 from integrated circuit substrate 60. Also, thermalisolation structure 50 provides an electrical interface between eachthermal sensor 40 in focal plane array 30 and integrated circuitsubstrate 60. The electrical interface allows integrated circuitsubstrate 60 to process signals based on incident infrared radiationdetected by focal plane array 30.

Mesa strip conductor 56 provides a signal path between the top of eachmesa-type structure 52 and the adjacent contact pad 62. Recommendedmaterials for the mesa strip conductors 56 include nickel chrome (NiCr)and titanium and tungsten alloys as well as other conductive oxides.

Various types of semiconductor materials and integrated circuitsubstrates may be satisfactorily used with the present invention. U.S.Pat. No. 4,143,269 entitled Ferroelectric Imaging System providesinformation concerning infrared detectors fabricated from pyroelectricmaterials and a silicon switching matrix or integrated circuitsubstrate. Examples of previous thermal isolation structures are shownin U.S. Pat. No. 5,047,644 entitled Polyimide Thermal Isolation Mesa fora Thermal Imaging System to Meissner, et al. The fabrication techniquesand the materials used in U.S. Pat. No. 5,047,644 may be used infabricating thermal isolation structures 50 and 150.

Bump bonding techniques may be satisfactorily used to form metal bondsbetween focal plane array 30 and thermal isolation structure 50. For oneembodiment of the present invention, bump bonding material 48 may beplaced on each sensor signal electrode 44 and bump bonding material 58may be placed on the top of each mesa strip conductor 56. Indium is oneexample of such bump bonding material.

The configuration of mesa-type structures 52 and the associated mesastrip conductors 56 are design choices, largely dependent upon thermalisolation and structural rigidity considerations. Alternativeconfigurations for mesa-type structures 52 include mesas with slopingsidewalls and mesas with vertical sidewalls. Mesa-type structures 52 ofthe present invention, including the exemplary thermal isolationstructure 50, for thermal imaging systems 20 and 120, may be fabricatedusing conventional photolithographic techniques.

Focal plane array 30 may be fabricated from multiple heterogeneouslayers formed on carrier substrate 70. One embodiment of the presentinvention includes combining a number of related techniques such as thinfilm growth, material formation, reticulation, and optical coatingprocesses to provide multiple heterogeneous layers corresponding withthe selected design and configuration of the resulting focal planearray. For some applications, only one or two layers of the desiredfocal plane are formed on carrier substrate 70. For other applications,all of the heterogeneous layers for the desired focal plane array may beformed on carrier substrate 70.

The various techniques may be integrated to allow fabrication of a focalplane array on carrier substrate 70 using processes associated with themanufacture of very large scale integrated circuits. Material usage andoverall process efficiency associated with fabricating a focal planearray may be substantially improved. For example, pyroelectric filmlayer 82 is preferably formed with approximately the same thickness asdesired for thermal sensitive elements 42. Thus, the possibility ofpolishing damage associated with previous techniques used to formthermal sensitive elements from pyroelectric materials have beensubstantially reduced or eliminated.

Various techniques may be used to form thin film layers 74, 34, 36, 82and 84 as shown in FIGS. 4 and 5. Often these techniques are dividedinto two groups--film growth by interaction of a vapor deposited specieswith an associated substrate and film formation by deposition withoutcausing changes to the associated substrate. The first group of thinfilm growth techniques includes thermal oxidation and nitridation ofsingle crystal silicon and polysilicon. The formation of silicides bydirect reaction of a deposited metal and the substrate is alsofrequently included in this first group of thin film growth techniques.

The second group of thin film growth techniques may be further dividedinto three subclasses of deposition. The first subclass is oftenreferred to as chemical vapor deposition (CVD) in which solid films areformed on a substrate by the chemical reaction of vapor phase chemicalswhich contain the desired constituents for the associated thin filmlayer. The second subclass is often referred to as physical vapordeposition (PVD) in which the desired thin film layer is physicallydislodged from a source to form a vapor and transport it across areduced pressure region to the substrate. The dislodged layer is thencondensed to form the desired thin film layer. The third subclasstypically involves coating the substrate with a liquid which is thendried to form the desired thin film layer. The formation of thin filmlayers by PVD includes such processes as sputtering, evaporation andmolecular beam epitaxy. Spin coating is one of the most commonly usedtechniques for depositing liquids on a substrate to form a thin filmlayer.

Thin film layers may also be satisfactorily grown in accordance with theteachings of the present invention by using techniques such as dipping,vapor phase deposition by sputtering, and sol/gel or metal oxidedecomposition (MOD) by spin coating. An important feature of the presentinvention includes selecting the desired process to establish thedesired electrical and thermal characteristics for the resulting focalplane array 30.

Carrier substrate 70 may be formed from silicon, ceramic alumina, orother suitable materials which are both chemically and thermallycompatible with the multiple heterogeneous layers which will be formedon surface 72 carrier substrate 70. For the embodiment of the presentinvention shown in FIGS. 2-6, carrier substrate 70 is preferably formedfrom ceramic alumina.

A layer of refractory material such as silicon dioxide, magnesium oxide,or calcium oxide may be deposited on surface 72 of carrier substrate 70to function as a release layer or separation layer which allows removingthe resulting focal plane array 30 from carrier substrate 70. As shownin FIGS. 2-6, carrier substrate 70 may be formed from ceramic aluminaand release layer or separation layer 74 formed from calcium oxide. Forother embodiments of the present invention, carrier substrate 70 may beformed from silicon and release layer or separation layer 74 formed fromsilicon dioxide.

For some applications, one or more barrier layers (not shown) may bedisposed between surface 72 of carrier substrate 70 and separation layer74. Such barrier layers may be used to establish chemical and/or thermalcompatibility between carrier substrate 70 and the various heterogeneouslayers formed thereon. Separation layer 74 is preferably formed from arefractory oxide or other material which is soluble in an etchantdifferent from the various etchants used to fabricate focal plane array30 from the multiple heterogeneous layers formed on carrier substrate70.

For the embodiment of the present invention shown in FIGS. 4-6, opticalcoating layer 34 is preferably formed on separation layer 74. Varioustypes of dielectric material such as zirconium dioxide may be used toform optical coating layer 34. The material selected to form opticalcoating layer 34 should preferably be stable during the various thermaland etching processes associated with fabricating focal plane array 30.The thickness of optical coating layer 34 may be adjusted based on therefractive index of the selected material. By varying the thicknessand/or the type of material used to form optical coating layer 34, theresulting focal plane array 30 may be tuned to maximize absorption ofincident infrared radiation over the desired spectrum. For someapplications, a thin layer of semi-transparent metal (not shown) may bedisposed between optical coating layer 34 and separation layer 74.

First layer 36 of electrically conductive material may next be depositedon optical coating layer 34. Various types of material such as nickel,nickel chrome, platinum, LSCO, ruthenium and/or ruthenium oxide may beused to form first electrically conductive layer 36. The type ofelectrically conductive material selected to form first layer 36 willdepend upon both the desired electrical conductivity, thermalconductivity, and chemical compatibility with the materials used to formthe other multiple heterogeneous layers on carrier substrate 70 asdesired for focal plane array 30. For the embodiment of the presentinvention shown in FIGS. 4-6, electrically conductive layer 36 may beformed from platinum.

Film layer 82 of the selected pyroelectric material may next be formedon electrically conductive layer 36. Various types of pyroelectricmaterial such as lead barium strontium titanate, barium strontiumtitanate, lead titanate, lead lanthanum titanate, lead zirconatetitanate, lead lanthanum zirconate titanate, lead strontium titanate,and lead scandium tantalate may be satisfactorily used depending uponthe desired operating characteristics for the resulting focal planearray. For the embodiment of the present invention shown in FIGS. 4-6,pyroelectric film layer 82 is preferably formed from barium strontiumtitanate. As will be explained later in more detail, low concentrationsof a second material may also be included with the barium strontiumtitanate to allow reducing the grain size at which the desiredproperties are obtained.

Various techniques may be used to form pyroelectric film layer 82. Forthe embodiment of the present invention shown in FIGS. 4-6, metalorganic deposition and spin coating along the previous discussed thinfilm growth techniques may be used to form pyroelectric film layer 82 onfirst electrically conductive layer 36. In comparison with the otherheterogeneous layers formed on carrier substrate 70 pyroelectric filmlayer 82 is relatively thick. The thickness of pyroelectric film layer82 is selected to correspond approximately to the desired thickness forthe resulting thermal sensitive elements 42. For some applications,thermal sensitive elements 42 will have a thickness of approximately 0.5micron to 20 microns, depending upon the desired thermal characteristicsor other compatible materials.

One aspect of the present invention may include the ability to form arelatively thick pyroelectric film layer 82 with controlled grain growthto achieve the desired pyroelectric properties for thermal sensitiveelements 42. Pyroelectric film layer 82 may be formed on carriersubstrate 70 using metal organic deposition techniques and liquidsolution or salts containing barium strontium titanate. A source of lead(Pb) may be added to lower the processing temperature. Small granules ofthe same composition may also be added to form nucleation sites. Thesmall granules will typically vary in size from one-half micron to twomicrons in diameter. The upper limit of two microns is based upon thedesired thermal and electrical characteristics associated with theresulting grained structure. Depending upon the type of materialselected to form pyroelectric film layer 82, the size of the smallgranules used to provide nucleation sites may be varied accordingly.

For some applications, additional doping materials may be includedwithin the liquid barium strontium titanate to provide the desiredoperating characteristics for the resulting thermal sensitive elements42. For other applications, pyroelectric film material may be formed byusing metal organic deposition and spin coating techniques which includea mixture of the selected pyroelectric material in both liquid andpowder form. The low concentrations of powder may be added to act as anucleation site or seed to support the desired grain growth.

Large grains frequently require a higher annealing temperature for theassociated film layer. By providing nucleation sites in the liquidsolution, satisfactory grain size is achieved to produce the desiredpyroelectric properties without requiring a high annealing temperature.By properly selecting the doping material and deposition rate, theannealing temperature for pyroelectric film layer 82 formed from bariumstrontium titanate may be reduced to between 300 to 800 degreescentigrade. Limiting the annealing temperature required for pyroelectricfilm layer 82, allows selecting a wider variety of materials to form theother heterogeneous layers on carrier substrate 70. As the annealingtemperature required for pyroelectric film layer 82 increases, fewermaterials are available to form layers 74, 34 and 36.

As best shown in FIG. 5, second layer 84 of electrically conductivematerial may next be deposited on pyroelectric film layer 82. Varioustypes of materials such as nickel, nickel chrome, platinum, LSCO,ruthenium and/or ruthenium oxide may be used to form second electricallyconductive layer 84. The type of electrically conductive materialselected to form second layer 84 will depend upon both the desiredelectrical conductivity, thermal conductivity, and chemicalcompatibility with the materials used to form the other multipleheterogeneous layers on carrier substrate 70. For the embodiment of thepresent invention shown in FIGS. 4-6, first electrically conductivelayer 36 and second electrically conductive layer 84 may be formed fromplatinum.

During the process of forming multiple heterogeneous layers 74, 34, 36,82, and 84 on carrier substrate 70, one or more low temperature heattreatments may be applied at selected steps in the fabrication process.By applying a low temperature heat treatment (50° C. to 500° C.),solvents may be evolved from the various layers on carrier substrate 70and some densification of the various layers will occur prior toreticulation of the individual thermal sensitive elements 42 and thefinal high temperature heat treatment of the selected pyroelectricmaterial. The number of low temperature heat treatments may be varieddepending upon the type of material selected for each layer 74, 34, 36,82, and 84 along with the process used to form the respective layers onsubstrate 70.

Following formation on carrier substrate 70 of the multipleheterogeneous layers for the desired focal plane array 30, variouspatterning and reticulation techniques may be used to provide thedesired number and configuration of thermal sensors 40 associated withthe resulting focal plane array 30. As best shown in FIGS. 5 and 6,etching techniques may be applied to second electrically conductivelayer 84 to form sensor signal electrodes 44. Pyroelectric film layer 82may then be etched with a different type of etchant to form thermalsensitive elements 42 associated with each thermal sensor 40. The typeof etchant and etching process will depend upon the material in therespective layers 74, 34, 36, 82, and 84. Following fabrication of thedesired thermal sensitive elements 42, an appropriate etchant may beapplied to dissolve separation layer 74 and allow removal of theresulting focal plane array 30 from carrier substrate 70. Depending uponthe particular fabrication processes being used, carrier substrate 70may be removed from focal plane array 30 either before or after focalplane array 30 has been mounted on thermal isolation structure 50 ofintegrated circuit substrate 60.

Thermal imaging system 120 shown in FIG. 7 may be described as a hybridsolid state system formed by mounting focal plane array 130 with thermalisolation structure 150 on integrated circuit substrate 160. Focal planearray 130 preferably includes a plurality of thermal sensors 140 whichmay be both electrically and mechanically coupled with integratedcircuit substrate 160 by thermal isolation structure 150. Thermalimaging system 20 and 120 have many components in common. The principaldifferences between thermal imaging system 20 and 120 result fromforming a plurality of slots 138 around the perimeter of each thermalsensor 140 to provide both thermal and electrical isolation betweenadjacent thermal sensors 140 in focal plane array 130.

Each thermal sensor 140 preferably includes an individual infraredabsorber assembly 132 having one or more layers of optical coating 134disposed on electrode 136. Optical coating 134 and electrode 136 may beformed on carrier substrate 70 using the same materials as previouslydescribed with respect to focal plane array 30. Since each thermalsensor 140 is isolated both electrically and thermally from adjacentthermal sensors 140, bias voltage must be supplied to each thermalsensor 140. For the embodiment of the present invention represented bythermal imaging system 120, integrated circuit substrate 160 preferablyincludes a common bus 164 which provides the desired bias voltage toeach thermal sensor 140.

Each thermal sensor 140 preferably includes a pair of thermal sensitiveelements 42 and 142 which may be formed from pyroelectric film layer 82as previously described. Bias voltage may be supplied to each thermalsensor 140 through the respective contact pad 162, mesa strip conductor156, bump bonding materials 58 and 48 to sensor signal electrode 144attached to thermal sensitive element 42. The bias voltage may besupplied from thermal sensitive element 142 to thermal sensitive element42 through electrode 136. Sensor signal electrode 44 and mesa stripconductor 56 are attached to thermal sensitive element 42 to provide asignal to integrated circuit substrate 60 in response to incidentinfrared radiation detected by the respective thermal sensor 140.

Focal plane array 130 may be formed on carrier substrate 70 using thefabrication techniques as previously described for focal plane array 30.When the fabrication process has reached the stage shown in FIG. 6,additional etchants may be applied to first electrically conductivelayer 36 and optical coating layer 34 to form a plurality of slots 138around the perimeter of each pair of thermal sensitive elements 42 and142. The resulting focal plane array 130 with separate individualthermal sensors 140 may then be mounted on integrated circuit substrate160.

The precise structural configuration and associated fabricationtechniques for focal plane arrays 30 and 130 are dependent upon theapplication chosen for the respective thermal sensor 40 and 140. Evenwithin a particular application, such as the exemplary thermal imagingsystems 20 and 120, numerous design choices will be routinelyimplemented by those skilled in the art.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of fabricating a focal plane arrayhaving a plurality of thermal sensors for use with a hybrid thermalimaging system comprising the steps of:forming a layer of opticalcoating on a carrier substrate; depositing a first layer of electricallyconducting material on the optical coating layer; forming a film layerof pyroelectric material on the first layer of electrically conductivematerial by mixing particles with liquid pyroelectric material toprovide nucleation sites for growing the film layer, wherein thepyroelectric material is selected from the group consisting of leadtitanate, lead lanthanum titanate, lead zirconate titanate, leadstrontium titanate, lead scandium tantalate, barium strontium titanate,and lead lanthanum zirconate titanate; forming a second layer ofelectrically conducting material on the pyroelectric film layer oppositefrom the first layer of electrically conducting material; and patterningthe pyroelectric film layer to correspond with the desired location andconfiguration of the associated thermal sensors.
 2. The method of claim1 wherein the step of forming the pyroelectric film layer furthercomprises the steps of:using metal organic deposition and spin coatingtechniques to deposit liquid pyroelectric material on the first layer ofelectrically conductive material; mixing particles of a selectedmaterial with the liquid pyroelectric material to provide nucleationsites for grain growth of the pyroelectric material; and spinning themixture of liquid and nucleation sites onto the carrier substrate toform the pyroelectric film layer.
 3. The method of claim 1 furthercomprising the steps of:forming a separation layer of calcium oxide onthe carrier substrate; forming the layer of optical coating frommaterial sensitive to infrared radiation on the separation layer;forming a first layer of platinum on the optical coating layer; forminga film layer of barium strontium titanate on the first layer ofplatinum; and forming a second layer of platinum on the barium strontiumtitanate layer opposite from the first layer of platinum.
 4. The methodof claim 1 wherein the carrier substrate comprises ceramic alumina. 5.The method of claim 1 wherein forming the film layer of pyroelectricmaterial further comprises the step of spin coating a liquid solution ofbarium strontium titanate having small granules of barium strontiumtitanate to provide nucleation sites to form the film layer.
 6. A methodof fabricating a hybrid thermal imaging system having a focal planearray with a plurality of thermal sensors mounted on a thermal isolationstructure projecting from an integrated circuit substrate, comprisingthe steps of:forming a separation layer of refractory material on acarrier substrate; forming a layer of optical coating material on theseparation layer; forming a first layer of electrically conductivematerial on the optical coating layer; forming a film layer ofpyroelectric material on the optical coating layer from a solution ofliquid pyroelectric material and powder nucleation sites by sol/gel andspin coating; forming a second layer of electrically conductive materialon the pyroelectric film layer opposite from the first layer ofelectrically conductive material; and patterning the pyroelectric filmlayer and the second layer of electrically conductive material tocorrespond with the desired location and configuration of the respectivethermal sensors.
 7. The method of fabricating a hybrid thermal imagingsystem as defined in claim 6 wherein the step of forming the film layerof pyroelectric material further comprises placing a liquid solution ofthe pyroelectrical material having small granules between approximately0.5 micron and 2.0 microns in diameter on the first layer ofelectrically conductive material.
 8. The method of fabricating a thermalimaging system as defined in claim 6 further comprising the stepsof:removing portions of the second layer of electrically conductivematerial to form sensor signal electrodes for each of the respectivethermal sensors; reticulating the pyroelectric film layer to form thedesired pattern of thermal sensors; and separating the focal plane arrayfrom the carrier substrate.
 9. The method of fabricating a thermalimaging system as defined in claim 6 further comprising the stepsof:forming a layer of bump bonding material on the second layer ofelectrically conductive material; removing portions of the bump bondinglayer and the second layer of electrically conductive material to formsensor signal electrodes with bump bonding material disposed thereon foreach of the respective thermal sensors; reticulating the pyroelectricfilm layer to form the desired pattern of thermal sensors; and bondingthe focal plane array with the thermal isolation structure projectingfrom the integrated circuit substrate using the bump bonding material onthe sensor signal electrodes.
 10. The method of fabricating a thermalimaging system as defined in claim 6 wherein the first layer and thesecond layer of electrically conductive material are selected from thegroup consisting of platinum, nickel chrome, ruthenium oxide andlanthanum strontium cobalt oxide.
 11. The method of fabricating athermal imaging system as defined in claim 6 further comprising thesteps of:forming the separation layer from calcium oxide; forming thefirst layer of electrically conductive material and the second layer ofelectrically conductive material from paltinum; removing portions of thesecond platinum layer to form sensor signal electrodes for each of therespective thermal sensors; reticulating the film layer of pyroelectricmaterial to form the desired pattern of thermal sensors; and separatingthe focal plane array from the carrier substrate.
 12. The method offabricating a thermal imaging system as defined in claim 6 furthercomprising the step of:forming the separation layer from silicondioxide; removing portions of the second layer of electricallyconductive material to form sensor signal electrodes for each of therespective thermal sensors; reticulating the film layer of pyroelectricmaterial to form the desired pattern of thermal sensors; mounting thefocal plane array on the thermal isolation structure projecting from theintegrated circuit substrate; and separating the focal plane array fromthe carrier substrate.
 13. A method of fabricating an array of thermalsensitive elements for use with a hybrid thermal imaging system, themethod comprising the steps of:forming a film layer of pyroelectricmaterial on a carrier substrate by mixing particles with liquidpyroelectric material to provide nucleation sites for growing the filmlayer; annealing the film layer at a temperature between 300 and 800degrees C.; and patterning the pyroelectric film layer to form thermalsensitive elements.
 14. A method of fabricating an array of thermalsensitive elements for use with a hybrid thermal imaging system, themethod comprising the steps of:forming a film layer of pyroelectricmaterial on a carrier substrate from a solution of liquid pyroelectricmaterial and powder nucleation sites; annealing the film layer at atemperature between 300° and 800° C.; and patterning the pyroelectricfilm layer to form thermal sensitive elements.
 15. The method of claim14, wherein the patterning step occurs before the annealing step. 16.The method of claim 14, further comprising at least one heat treatmentstep before the annealing step; wherein the temperature of the heattreatment step is between 50 degrees C. and 500 degrees C.